Electrolytic condenser



Jan. 25, 1938. F. KELLER ET AL ELECTROLYTIC CONDENSER Filed March 25,1937 Cu 6 R W WY 2? W N L 5 E A O flm D5 55 an m C Y B Patented Jan. 25,1938 Pennsylvania ELECTROLYTIC CONDENSER Fred Keller and Charles M.Hastings, New Kensington, Pa., assignors to Aluminum Company of America,Pittsburgh, Pa., a corporation of Application March 23, 1937, Serial No.132,527

11 Claims.

This invention relates to the manufacture of electrolytic condensers andit is particularly concerned with the type wherein aluminum isemployedas the metallic conductor.

Aluminum has been extensively used in making both the so-called wet anddry electrolytic condensers because of its desirable film formingcharacteristics. When so used, the metal, in the form of sheet or foil,is generally folded or wound in such a manner as to provide a largesurface area within a small space and thus increase the capacity of 'thecondenser. It has been further found that the effective surface area ofthe aluminum can be considerably increased by etching the metal beforeforming the dielectric film on its surface. This chemical attack on themetal leaves a roughened surface which in turn serves to increase thecapacity of the condensers made from such foil or sheet. By means ofthis improvement it is possible to reduce the quantity of metal neededfor a condenser of a given capacity, or to increase its capacity if thesame amount of sheet or foil is to be employed. This improvement hasbeen of particular benefit in the manufacture of the so-called pastetype of condensers where an absorbent material containing an electrolyteis placed between the sheets of aluminum. The aluminum commonly used formaking these condensers contains less than 0.2 per cent total of theimpurities iron, silicon and copper.

It has been discovered that certain irregularities in the capacity ofetched aluminum foil condensers sometimes occur which may be traced to anon-uniformity in the etched surface. This non-uniformity may be theresult of variations in the foil or the method of etching. Since thefoil is produced in strips of considerable length, and these strips arepassed through an etching medium, it is not surprising that there shouldbe some variation in the surface condition over the entire length of acoil. This non-uniformity, as reflected in the difference in capacity ofthe condensers formed from such foil, is obviously undesirable and to beavoided if possible. Our invention is directed to changing thecharacteristlcs of the metal .in the form of foil to render it moreuniformly susceptible to attack by the etching agent and to produce amaximum increase in the effective surface for a given amount of chemicalattack. The principal object of our invention is to provide a method oftreating aluminum sheet which will insure a uniform etching of themetal. Anotherobject is to provide a method of treating aluminum sheetwhich will create a condition in the metal whereby a maximum rougheningof the surface can be secured by etching with a minimum removal ofmetal. still another object is to provide a practical method forpreparing aluminum foil that is to be etched and used in makingelectrolytic condensers.

Our invention is based upon the discovery that aluminum sheets and foilwhich have been heated at-650 to 950 F. for a period of a few minutes upto 12 hours, depending on the rapidity with which they are brought up totemperature, cooled to room temperature and then reheated at 250 to 400F. for a period of 6 to 36 hours and finally cooled to room temperature,can be etched uniformly and deeply with a minimum removal of metal. Analternative treatment consists in slowly cooling the metal in thefurnace from the high temperature heating range to below 400 F. over aperiod of to 25 hours at a rate of about 25 F. per hour. After reachinga temperature below 400 F. the coiled sheet or foil can be removed fromthe furnace and cooled to room temperature. The thermal treatmentaccording to either method serves to first dissolve any solubleimpurities and then cause at least a partial precipitation of thedissolved constituents. We have found that the distribution andcharacter of the undissolved particles of iron, silicon and copperimpurities greatly affect the etching characteristics of the aluminum.We have found, for example, that an untreated strip of foil etchesunevenly, and that although the surface may be roughened by the chemicalattack yet the attack is not uniform, there being relatively deep pitsin some places and only shallow pits in other areas. By way of contrast,the foil which has been thermally treated is uniformly attacked, thatis, the pits are of nearly the same depth, and only a small amount ofthe aluminum matrix is dissolved. The uniform distribution of theprecipitated particles appears to materially affect the etchingcharacteristics of the foil, and aids in securing a maximum rougheningof the surface with a minimum. removal of the aluminum.

A condenser of the type wherein etched aluminum foil is employed isillustrated in the accompanying figure. This condenser is of the pastetype in which an absorbent material containing the electrolyte is placedbetween the metal sheets. Referring to the drawing, l is a containermade from a suitable material such as aluminum, and closed at the openend with insulating material 2 formed in such a manner as to provide athreaded projection for attaching the condenser to a panel or othermounting. Within the container is mounted the-condenser itself whichconsists of one or more pairs of aluminum foil strips 3 with aninterposed absorbent separator or sheet 4 of gauze, paper, or the like.In a D. C. condenser only one strip need be etched since it serves asthe anode. The absorbent separator, impregnated with a suitable viscouselectrolyte, should cover the total area of the two layers of foil toobtain maximum capacity. It is convenient to assemble this structure inthe form of a long strip of previously etched and electrolyticallyfilmed foil which is rolled into a substantially cylindrical shape asindicated, with another absorbent strip 5 between the successive turnsthereof. A terminal strip 6 is formed at the end of one layer of foiland another terminal strip i is formed at the end of the other layer offoil; the terminal strip 0 being the anode foil in a D. C. condenser. Anelectrical connection with the anode foil is provided by the rod 3extending through the insulating cap 2. The other layer of foil merelyfurnishes an auxiliary conducting path and connection to the electrolytecathode. A suitable cathode terminal 9 may be provided as shown.

The aluminum foil or sheet to be used in making the condenser may berolled to the desired gauge according to the usual fabrication methodsand wound into coils. Where the sheet is handled in the form of coils,they may be heated in an annealing furnace for the required length oftime and then either cooled in the furnace to below 400 F. or removedfrom the furnace and cooled to room temperature. In the latter case thecoils are generally placed in another furnace operating at a temperaturebetween 250 and 400 F. and heated for the necessary length of time. Thetime required to heat a particular size of coil or load of coils is amatter for experimental determination, but in any case the initialheating at 650 to 950 F. should occupy from 1 to 12 hours. In ordinarycommercial operations we prefer to hold the metal at 800 to 850 F. forone to two hours. The method of cooling the metal from the elevatedtemperature is a matter to be determined by economic considerationsinasmuch as the same effect is obtained whether the metal is cooled inthe furnace or removed from the furnace and later reheated. Where thecooling is to be done in the furnace, the length of time required toreach 400 F. or lower depends upon the furnace temperature and the massof metal being heated. Generally from 5 to 25 hours is needed for thispurpose, the rate of cooling varying between 25 and 50 F. per hour.After reaching a temperature of 375 to 400 F. the metal can beremovedfrom the furnace and allowed to cool to room temperature.

Although it is common practice to heat the foil in coiled form, yet itis possible to obtain the same result by passing the foil in strip formthrough a suitably designed continuous furnace. Under such conditionsthe desired effect of heating may be obtained in 5 minutes instead of anhour or longer as where a coil is being treated. In any event, the hightemperature treatment should serve to dissolve all the solubleimpurities, that is, substantiallyeliminate irregularities in thestructure of the metal resulting from working as well as equalize thedistribution of soluble impurities. The period required to bring aboutthis condition may thus vary between 5 minutes and 12 hours dependingupon the mass of metal being heated at one time.

If the foil is treated in strip form, it is most convenient to apply thesecond thermal treatment by passing the strip through a second furnaceoperating at a temperature of 250 to 400 F. The period of exposureshould be long enough to cause at least a. partial if not a completeprecipitation of the dissolved impurities.

Where the coiled sheet is removed from the furnace immediately after theannealing treatment, it is to be cooled relatively rapidly to about roomtemperature, at least below 250' 1"., and

arcane then reheated to 250 to 400 F, for a period of 6 to 36 hours.Upon completion of this low temperature treatment, the metal is to beremoved from the furnace and cooled to room temperature.

After either of the foregoing thermal treatments the coil, now at roomtemperature, is unwound and passed through an etching solution. It isobvious that any dirt or adhering foreign matter should be removed fromthe metal prior to etching in order to insure uniformity of attack. Anumber of substances are known which will attack aluminum but forcommercial purposes either strong acid or caustic solutions arepreferred. For the treatment of aluminum for electrolytic condensers, anetching with an aqueous solution of hydrochloric acid isusuallypreferred. An 11 per cent by weight solution of hydrochloric acid whichis commonly used for etching aluminum for other purposes is verysatisfactory for treating the metal intended for the manufacture ofcondensers. The sheet or foil is generally etched on both sides sinceboth sides are active in the operation of the condenser. Where foil isto be treated, it is obvious that the time of exposure to the etchingsolution must be brief and closely regulated in order to avoid an unduereduction in thickness of the metal sheet.

The aluminum employed for making electrolyti c condensers should be of acommercial purity having a maximum of 0.6 per cent total of theimpurities, iron, silicon and copper. However, a maximum of 0.2 per centis preferred, because in electrolytic condensers the impurities may be asource of electrical leakage and reduce the power factor. A high leakagein condensers oi the paste type referred to hereinabove tends to shortentheir useful life and it is therefore to be minimized as much aspossible.

An illustration of the influence of a properly prepared and etchedaluminum foil upon the capacity of a condenser is to be found in thefollowing examples. The same material in the form of foil 0.004 inch inthickness was used in each test. The aluminum had a total iron, siliconand copper impurity content of 0.19 per cent. One group of sheetsamples, designated A, was annealed at 500 F. for 1 hour, andimmediately cooled to room temperature. A secorYd group, B, was annealedat 800 F. for 1 hour and immediately cooled to room temperature. A thirdgroup, C, was annealed at 800 cooled to room temperature and reheated to350 F. for 36 hours. All of the samples were etched in an 11 per cent byweight aqueous solution of hydrochloric acid at a temperature of 65 F.for 1 minute. The electrical capacity of these samples was thendetermined on a Wien bridge using a 60 cycle alternating current. Theaverage capacity in microfarads of the three groups of samples is givenbelow.

Thermal treat- Capacity Group ment microiamds 9. 36 8m F. for 1 hr. 10.38 800F.iorlhr.+

300 1".ior36hrs. 13.30

The improvement gained through treating the foil in accordance with ourinvention is at once apparent from these typical results. It may be seenthat it is not only necessary to heat the foil within the limits of 650to 950 F., but that a subsequent thermal treatment is essential to developing the best capacity.

Having thus described our invention and the manner in which it is to beperformed,

We claim:

1. In the art of making electrolytic condensers from aliuninum sheet,the method of rendering said sheet uniformly etchable comprising heatingthe sheet at 650 to 950 F. for 5 minutes to 12 hours, cooling the sheetto room temperature, reheating it to 250 to 400 F. for 6 to 36 hours,and finally cooling to room temperature.

2. In the art of making electrolytic condensers from'aluminum sheet, themethod of rendering said sheet uniformly etchable comprising heating thesheet at 650 to 950 F. for 1 to 12 hours, cooling the sheet to roomtemperature, reheating it to 250 to 400 F. for 6 to 36 hours, andfinally cooling to room temperature.

3. In the art of making electrolytic condensers from, aluminum sheet,the method of rendering said sheet uniformly etchable comprising heatingthe sheet at 650 to 950 F. for '5 minutes to 12 hours, rapidly coolingthe sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36hours, and finally cooling to room temperature.

4. In the art of making electrolytic condensers from aluminum sheet, themethod of rendering said sheet uniformly etchable which contains a totalof not more than 0.6 per cent of the impurities iron, silicon andcopper, said method comprising heating the sheet at 650 to 950 F. for 5minutes to 12 hours,

temperature, reheating it to 250 to 400 F. for 6 to 36 hours, andfinally cooling to room temperature.

5. In the art of making electrolytic condensers from aluminum sheet, themethod of rendering said sheet uniformly etchable which contains a totalof not more than 0.6 per cent of the impurities iron, silicon andcopper, said method comprising heating the sheet at 650 to 950 F. for 1to 12 hours, cooling the sheet to room temperature, reheating it to 250to 400 F. for 12 to 36 hours, and finally cooling to room temperature.

6. In the art of making electrolytic condensers from aluminum sheet, themethod of rendering said sheet uniformly etchable which contains a totalof not more than 0.6 per cent of the impurities iron, silicon andcopper, said method comprising heating the sheet at 650 to 950 F. for 5minutes to 12 hours and thereafter subjecting said sheet to a thermaltreatment between 250 and 400 F, followed by'cooling to roomtemperature.

cooling the sheet to room 7. In the art of making electrolyticcondensers from aluminum sheet, the method of rendering said sheetuniformly etchable which contains a total of not more than 0.2 per centof the impurities iron, silicon and copper, said method comprisingheating the sheet at 650 to 950 F. for 5 minutes to 12 hours, coolingthe sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36hours, and finally cooling to room temperature.

8. In the art of making electrolytic condensers from aluminum sheet, themethod of rendering said sheet uniformly etchable which contains a totalof not more than 0.2 per cent of the impurities iron, silicon andcopper, said method comprising heating the sheet at 800 to 850 F. for 1to 2 hours, cooling the sheet to room temperature, reheating it to 350to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

9. In the art of making electrolytic condensers from aluminum sheet, themethod of rendering said sheet uniformly etchable comprising heating thesheet at 650 to 950 F. for 1 to 12 hours, slowly cooling it in thefurnace to below 400 F. within a period of 5 to 25 hours at a rate of 25to F. per hour, removing the sheet from the furnace and cooling it toroom temperature.

10. In the art of making electrolytic condensers from aluminum sheet,the method of rendering said sheet uniformly etchahle which contains atotal of not more than 0.6 per cent of the impurities iron, silicon andcopper, said method comprising heating for l to 12 hours, slowly coolingit in the furnace to below 400 F. within a period of 5 to 25 hours at arate of 25 to 50 F. per hour, removing the sheet from thefurnace andcooling it to room temperature.

11. In the art of making electrolytic condensers from aluminum sheet,the method of rendering said sheet uniformly etchable which contains atotal of not more than 0.2 per cent of the impurities iron, silicon andcopper, said method comprising heating the sheet at 650 to 950 F. for

the sheet at 650 to 950 F.

1 to 12 hours, slowly cooling it in the furnace to below 400 F.'wlthin aperiod of 5 to 25 hours at a rate of 25 to 50 F. per hour, removing thesheet from the furnace and cooling it to room temperature. FRED KELLER.

CHARLES M. HASTINGS.

